This invention relates generally to high-voltage generation systems for use with X-ray tubes and, more particularly, to a circuit for limiting the current flow in an X-ray generator system.
It is common in the generation and use of X-rays to select a particular voltage and current level to fit the particular application or procedure at hand. For example, in the field of medical X-ray imaging, a typical voltage level that may be applied in conventional radiography might be in the range of 50 kv to 150 kv, whereas in a fluorographic application the voltage is more likely to be in the 50 kV to 120 kV range, and for use in mammographic X-ray applications it is more likely to be in the range between 24 kV and 50 kV. Similarly, the level of current which is applied may vary from 0.1 mA for fluorographic applications to 1250 mA for certain radiographic procedures. Traditionally, these voltage and current levels have been controlled by the use of circuit design features which allow the operator to preset the desired kVp and mA settings that were desired. Because of system variations which can occur during an exposure, such as, for example, a change in the load, a change in the line voltage, or a change in the filament temperature, it has been impossible to precisely maintain the kVp and mA values at the preferred levels. The manufacturers of X-ray generators have traditionally tried to anticipate the changes that may occur and to incorporate circuit design features which would compensate for those variations in a manner sufficient to hold the kVp and mA within predetermined tolerances.
Recent developments have occurred along the lines of a closed-loop feedback system which would overcome the disadvantages of the open-loop system discussed above. One such system is that for a closed-loop feedback system to control mA in an X-ray generator system. This system is described in U.S. patent application No. 375,088, filed on May 5, 1982 and assigned to the assignee of the present invention.
In the area of kVp control, there has been no development of a satisfactory closed-loop system which senses the output voltage and uses that feedback signal to directly modulate the output voltage in a quick, effective, and responsive manner to maintain a predetermined voltage level.
The conventional approach for maintaining a substantially constant voltage level with the variations that may occur in the line is to use the so-called volt-pac which is a variable input/output autotransformer driven by a motor to obtain a variable output. A primary disadvantage of the volt-pac is that it is relatively slow in operation, i.e., the volt-pac has a response time of about 1 second. For this reason, the volt-pac control is used only to set up the correct voltages at the start of exposure and is not thereafter adjusted except during long (i.e., fluoroscopic) exposures. This is to be compared with a desired response time in the millisecond range for an X-ray generator system which is able to provide a short power pulse of good definition for a large variety of procedures and applications. For example, it is desirable to have a high-voltage pulse with a very quick rise time, i.e., as short as 1 millisecond, a flat peak for an exposure as short as 1 millisecond, and a quick fall time. Hence, corrections need to be accomplished in less than one millisecond.
The use of an inverter in an X-ray generator circuit to provide an alternating current to the primary of a high-voltage transformer is known. However, due primarily to the fact that they are relatively difficult to control, transistors have generally not been used for this purpose. Rather, it has been the thyristor which has been used for the switching device in these applications. Although thyristors are considered to be generally rugged and relatively easy to control, they have the inherent disadvantage of requiring the use of forced commutating circuitry. Thus, not only is there a need for extra components, but also, the added capacitance tends to substantially slow down the response time of the circuit. For example, when using a thyristor inverter, it would be difficult to obtain a short, high-voltage pulse in the range of 1 msec. duration while at the same time maintaining a reasonable level of reproducibility.
For the control of the a.c. output from an inverter, there are a number of possible techniques for controlling the d.c. voltage supply to the inverter: phase-controlled rectifiers, transistor series or shunt regulators; and semi-conductor switching-type d.c. voltage controls, to name a few. Of these, the semi-conductor switching device commonly known as the chopper can generally provide more efficient and faster response d.c. voltage controls than the other techniques. However, because of the substantial filtering requirement in the d.c. circuit, it is much too slow in response time for operation in a complete closed-loop voltage regulated inverter power supply. With such an indirect approach, there are further circuit losses that are caused by the forced commutation circuitry that must be used to accommodate the large voltage and current variations that are necessary in the operation of an X-ray generation system. Moreover, with such an arrangement, it will be recognized that the power delivered by the inverter is handled twice, once by the d.c. voltage control and once by the inverter.
In addition to the inherent variations that occur in the source and in the load, there are also certain, occasional, unplanned conditions, such as an arc in the tube, which occur on the high voltage side which, if not controlled, may lead to harm to the components. Further, in any control network, there is a possibility of malfunction or failure in the low voltage control circuitry which, if not detected and attended to, may cause undesirable consequences at the output or within the control network itself. Thus, with any control or performance-enhancing features that may be added to a conventional system, there are related monitoring and regulating features which must be provided to accommodate these enhancements. Accordingly, in the field of X-ray generators for use in medical diagnostic equipment, there has been a reluctance to introduce any significant change to conventional systems.
Although it has long been the desire of X-ray-generation manufacturers to provide a closed-loop voltage feedback system, the typical requirements for X-ray applications (i.e., variable loads in the range of 0.1-1250 mA, variable voltages in the range of 24-150 kVp, and mAs as low as 0.25), a suitable such system has been difficult to make. The task is made more difficult by the various performance requirements such as good ripple control, high reproducibility, good linearity, and a controlled shape of the power waveform with a fast rise time, a steady-state, short exposure time, and a short fall time.
In addition to the occurrence of over-voltages in the system, it will be recognized that there may also be excessive current levels in the system which could cause damage to the X-ray tube itself or to the high-voltage or low-voltage components within the system. When using an inverter in an X-ray generator system, it is desirable to have as much current as possible going through the switching devices. However, in order to maintain a reasonable life for those switching devices, it is imperative that the current level not exceed a predetermined level. The function of determining when the current level is coming close to its limit and for taking the necessary action to prevent that occurrence can be difficult to accomplish. This is especially true when an inverter is operating at higher frequencies.